2.1.1 Shunt Resistor Setup

The shunt resistor may be placed on either side of the load to be measured. A high-side measurement is performed if one of the shunt sides connect to the high power rail V_CC. If the shunt connects to ground, low-side measurement is used. These techniques both have advantages and disadvantages.

High-side measurements have the advantage that the load is still connected directly to ground. Because of the physical nature of most PCBs, including ground planes close to paths and ground-referenced component casings, a short to ground is more likely to happen than one to a different voltage. If the load shorts to ground with low-side measurements, the short’s high current flows around the current measurement shunt resistor, and the short may not be measurable. See Figure 2-1. As shown in Figure 2-2, with high-side measurement, a load short will lead to higher current readings at R_SHUNT and can be detected.

Low-side measurements have the advantage of low common mode voltages. As long as the ADC is referenced to the same ground as the circuit it measures, the voltage reading of the load side of the shunt will represent the entire voltage drop. With high-side measurements, on the other hand, both sides of the shunt resistor will be measured to more than 0V, and the voltage difference reflects the current. Running differential conversions lets the ADC sample only the difference between the voltages and utilize the full range of the ADC with 0V corresponding to 0A of current.